1. Field of the Invention
The present invention relates to a mobile terminal, and more particularly, to a mobile terminal and controlling method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for reducing fluctuation of a screen.
2. Discussion of the Related Art
A camera is configured to photograph a subject in a manner of applying an image to an image pickup unit by letting a light enter a lens momentarily. And, applied fields of the camera tend to increasingly expand. It does not come as a surprise that a camera is located on such a mobile terminal as a smartphone, a tablet PC and the like. As cameras are widely used for various purposes, they are currently used for various fields including a security device (e.g., a vehicle black box, a CCTV, etc.) and the like.
In order to obtain a clear photographed image of a subject through a camera, a lens should be focused on a subject in a manner of adjusting a position of the lens. Generally, in order to adjust a position of a lens in a camera, the camera should consist of an image pickup unit, the lens, a lens drive unit for adjusting the position of the lens and a control unit for applying a control signal to the lens drive unit. In particular, the control unit is able to control a focus in a manner of adjusting the position of the lens by applying the control signal to the lens drive unit and placing the lens at a point for maximizing a sharpness of a subject in an image formed on the image pickup unit.
Yet, since a lens should be shifted back and forth in an optical axis direction in order to control a focus, a view angle of the lens keeps changing in the course of focusing. Due to the view angle change of the lens, a size of a subject keeps changing in a screen provided to a user. For instance, when a focus is adjusted by shifting a lens of a camera in an optical axis direction, FIG. 1 is a diagram for one example to describe a process for changing a view angle of the lens and a change of a screen provided to a user in due course. In each of FIGS. 1 (a) to 1 (d), one drawing shown in a top part is a diagram for one example to describe a focal distance between a lens and an image pickup unit 20, while the other drawing in a bottom part is a diagram for one example to describe an image provided to a user.
First of all, referring to FIG. 1 (a), assume that an initial state of a lens is a state in which the lens is spaced apart from the image sensing lens 20 by d1. In the state shown in FIG. 1 (a), in case of attempting to focus on a specific subject 1, a controller measures a sharpness of the subject 1 imaged in an image pickup unit 20 on each movement of a lens while adjusting a distance between the lens and the image pickup unit 20. In particular, the controller is able to measure the sharpness of the subject 1 by moving the lens in an optical axis direction (i.e., an axial direction corresponding to a direction of a light incident through the lens). In particular, if the lens is moved, a distance between the image pickup unit 20 and the lens further increases or decreases.
For instance, the controller can measure the sharpness of the subject 1 by controlling the lens to get away from the image pickup unit 20 gradually. When the lens is spaced apart from the image pickup unit 20 by d2, assume that the sharpness of the subject 1 has a greatest value. On this assumption, if the distance between the lens and the image pickup unit 20 increases to d2 from d1 gradually, the sharpness of the subject 1 will increase gradually. Thereafter, if the distance between the lens and the image pickup unit 20 increases to d3 from d2, since the sharpness of the subject 1 will decrease. Therefore, the controller can determine that the lens is focused on the subject 1 if the distance between the lens and the image pickup unit 20 is d2. In particular, while the controller increases the distance between the lens and the image pickup unit 20 gradually [FIG. 1 (a) FIG. 1 (b) FIG. 1 (c)], if the sharpness of the subject starts to decrease [FIG. 1 (b) FIG. 1 (c)], the controller determines that a focal distance is passed. Therefore, the controller can decrease the distance between the lens and the image pickup unit 20 again [FIG. 1(c) FIG. 1 (d)].
Yet, in case of adjusting a focus on the subject 1 by moving the lens, a view angle of the lens changes continuously. The change of the view angle of the lens enables a photographable rage of a camera to vary gradually. For instance, referring to FIG. 1 (a) and FIG. 1 (b), if the view angle of the lens further decreases, a light can be received for a smaller space. Hence, a size of the subject 1 in a preview image outputted through a display unit can be changed [cf. size changes of a tree shown in FIGS. 1 (a) to 1 (d)]. In particular, a size of the subject 1 in a preview image created from targeting a wide space may be different from a size of the subject 1 in a preview image created from targeting a narrow space. Hence, referring to FIG. 1 (a) to FIG. 1 (d), the size of the subject 1 in an image provided to a user can be momentarily enlarged or reduced.
However, as a size of a subject momentarily increases or decreases, a user may have an impression as if a screen fluctuates. Particularly, if a video file is created by taking a moving object as a subject, since a focus should be adjusted in response to each movement of the object, the screen fluctuation due to the increasing/decreasing size of the subject may be recognized as a serious problem.